Study of boron removal from molten silicon by slag refining under atmosphere

Solar energy is expected to be in great demand within the next few years. In relation to this, a global shortage in the solar-grade silicon (SoG-Si) used in the production of solar cells has motivated numerous studies on metallurgical-grade Si (MG-Si) refining. However, improvements in this material are limited by the difficulties involved in reducing boron (B) and phosphorus content. The present study investigated the removal of B using (CaF2)-Al2O3-CaO-SiO2 and Na2SiO3-CaO-SiO2 slag in an ambient air atmosphere in order to refine MG-Si. The mass transfer coefficients of B in molten Si (kd[B]=1.19×10−4 cm s−1) and BO1.5 in molten slag (kd(B3+)=1.01×10−5 cm s−1) were deduced through kinetic analysis. The mass transfer process of boron oxide limited slag refining; however, secondary slag treatment effectively improved the efficiency of B removal. LB was determined by the combined effect of the oxygen partial pressure and the molten slag structure.